Many aircraft braking systems incorporate carbon-carbon composite discs. The brakes can absorb large amounts of kinetic energy required to stop the aircraft during a landing. During some of the stops, the carbon may be heated to sufficiently high temperatures that may oxidize the surfaces exposed to air. Carbon composites with the thermal and mechanical properties required for specific brake designs have been prepared. However, these composites may have residual open porosities of about 5% to 10%. The open pores allow air, moisture and contaminates to infiltrate into the carbon-carbon composite. At the elevated temperatures reached during use, the infiltrate materials may cause or enhance internal oxidation of the carbon-carbon composite. The internal oxidation may weaken the carbon-carbon composite, especially in and around the brake rotor lugs or stator slots. These areas transmit the torque during braking. One simple, low-cost method of minimizing loss of strength and structural integrity is the application of phosphoric acid to non-wear surfaces of brake discs, followed by baking to 650° C. This method inhibits oxidation of carbon sufficiently for many applications, including aircraft brakes.
Similarly, carbon-carbon composites have been coated with barriers to include silicon-based coatings, such as silicon carbide. The barriers may reduce the inflow of air, and thereby inhibit oxidation of the carbon-carbon composites. Unfortunately, the barriers may crack too easily, and may have an undesirable inherent porosity. These cracks and pores allow air to infiltrate the carbon-carbon composite despite the presence of the barrier.
Some commercial transport brakes have suffered crushing in the lugs or stator slots. The damage has been associated generally with oxidation of the carbon-carbon composite at elevated temperatures. A specific association is damage caused by the oxidation enlargement of cracks around fibers, or enlargement of cracks in a barrier coating applied to the carbon-carbon composite. The enlargement effect may occur at depths of up to about 12.5 millimeters (mm) (0.5 inch) beneath exposed surfaces.
Elements identified in severely oxidized regions include potassium (K) and sodium (Na). Alkali elements are believed to catalyze carbon oxidation and such oxidation catalysts are generally present as contaminants. These contaminating materials may come into contact with a carbon-carbon composite, such as an aircraft brake, during cleaning and/or de-icing procedures used on aircraft. Such procedures can use cleaning or de-icing materials that include alkali metals (such as sodium and potassium). Other sources of sodium include salt deposits left from seawater or sea spray. These contaminating materials may penetrate into the pores of the carbon-carbon composite aircraft brakes, leaving catalytic deposits within the pores. When such contamination occurs, the rate of carbon loss by oxidation can be increased by one to two orders of magnitude. It may be desirable to provide protection against such catalyzed oxidation.
McKee points out that phosphates can deactivate catalytic impurities in carbon by converting them to inactive, stable phosphates (D. W. McKee, Chemistry and Physics of Carbon 16, P. L. Walker and P. A. Thrower eds., Marcel Dekker, 1981, p. 30). Woodburn and Lynch (U.S. Pat. No. 2,685,539) describe ways of impregnating pores in carbon or graphite with aluminum phosphate. Woodburn and Lynch disclose suitable compositions having a molar ratio of Al2O3:P2O5 in a range of from about 0.2:1 to about 0.8:1.
Other oxidation inhibition methods previously used are disclosed in: U.S. Pat. No. 4,439,491, issued to Wilson, which relates to carbon protected against oxidation by application of an aqueous solution comprising mono-ammonium phosphate, zinc orthophosphate, phosphoric acid, boric acid, cupric oxide, and wetting agent; U.S. Pat. No. 4,837,073, issued to McAllister et al, which relates to a barrier coating and penetrant providing oxidation protection for carbon-carbon materials; U.S. Pat. No. 5,401,440, issued to Stover et al, which relates to a composition for inhibiting catalyzed oxidation of carbon-carbon composites that includes an oxidation inhibiting composition comprising phosphorus acid, a metal phosphate, and a compatible wetting agent; and, U.S. Pat. Nos. 5,759,622, issued to Stover, which relates to a composition for inhibiting catalyzed oxidation of carbon-carbon composites that includes an oxidation inhibiting composition comprising phosphoric acid, a metal phosphate or combination of a zinc salt and an aluminum salt, and a compatible wetting agent. The carbon-carbon composite in Stover may have a barrier coating.
More recent patents, including U.S. Pat. No. 5,714,244, issued to Delval et al., discuss problems associated with the sensitivity of certain phosphate inhibitor systems to absorption of moisture. This can reduce the friction performance of brake materials after exposure to certain environments. The composition described in U.S. Pat. No. 5,759,622 may be vulnerable to moisture absorption. This invention provides a solution to this problem.